Explanation:
On September 29, this magnificent eruptive solar
prominence lifted away from
the Sun's surface,
unfurling into space over the course of several hours.
Suspended in twisted
magnetic fields,
the
hot plasma structure is many times
the size of planet Earth and
was captured in this view by the Sun-watching
STEREO
(Ahead) spacecraft.
The image was recorded in extreme ultraviolet light emitted
by ionized Helium, an element
originally identified in the solar spectrum.
Seen against the brilliant solar surface in visible light,
such prominences appear as dark
filaments because they are relatively cool.
But they are bright themselves when viewed
against the blackness of space, arcing above the Sun's edge.
A video of the eruption (a 2.6MB .mov file) is
available here.